About 40% of the nation's 4-year olds now attend either a state-funded pre-K or Head Start program. These high rates of participation in formal early childhood programs represent a national experiment focused on finding the best means of launching all young children on a trajectory of school success. Empirical work on pre-K education is thus directed to two core questions. First, under what conditions does pre-K education produce gains in children's cognitive and social development? Second, for whom does pre-K education produce such gains? From the perspective of developmental science, answers to these questions will inform enduring questions about beneficial environmental influences on early life trajectories (Shonkoff & Phillips, 2000). From a policy perspective, answers to these questions will guide efforts to focus public resources on program elements and children for whom strong impacts are most likely. The proposed research will address these questions in the context of Tulsa, Oklahoma's universal pre-K program, which has generated strong impacts across racial and income groups in three cohorts of children (Gormley & Gayer, 2005; Gormley, Gayer, Phillips, & Dawson, 2005; Gormley, Phillips, & Gayer, 2008). Replication analyses will be run using the 11-state NCEDL/SWEEP prekindergarten dataset to compare findings from the Tulsa pre-K program to a larger set of state pre-kindergarten programs. Using both a regression discontinuity design and propensity score matching, this study will address two aims. The first aim is to identify the specific aspects of children's experiences in pre-K and Head Start classrooms that are associated with cognitive and social-emotional gains at kindergarten entry. The contribution of classroom instructional and emotional climate, time on math and literacy instruction, patterns of curricula use, and teacher characteristics will be examined. This aim will inform pressing policy questions about the elements of pre-K education that contribute most strongly to preparing young children for school and thus guide efforts to direct resources to the active ingredients of preschool programs. The second aim is to examine the impact of pre-K education for children who differ in special education status on cognitive and social-emotional functioning at kindergarten entry. This aim will supplement available knowledge about differential preschool program impacts on subgroups of children defined by demographic features (e.g., race-ethnicity and income) with information on this additional subgroup of children who, as with children in poverty, are a focus of public investments in early education.